1. Field of the Invention
The present invention relates to a process for preparing crystalline zeolites having the MEL structure using an organic structure-directing agent (xe2x80x9cSDAxe2x80x9d) comprising a 2,2-diethoxyethyltrimethylammonium cation.
2. State of the Art
ZSM-11, a zeolite having the MEL structure, was first prepared by Kokotailo et al. in 1978 using tetrabutylammonium (TBA) as the organic structure-directing agent (SDA). (See Kokotailo et al., Nature, 1978, 275, 119.) Soon after, it became apparent that the material made by the TBA-mediated synthesis was actually a ZSM-11/ZSM-5 intergrowth, and that pure phase MEL was quite difficult to prepare. In 1994, Nakagawa et al. provided the first synthesis procedure for pure-phase MEL, utilizing 3,5-dimethylpiperidinium derivatives as SDAs (see WO 95 09812).
The present invention involves the use of an oxygen-containing SDA to prepare MEL zeolites. While there have been previous reports of oxygen-containing SDAs, such as 1,3,5-trioxane (see Keijsper et al., Zeolites: Facts, Figures, Future, 1989) or crown ethers (see French Patent No. 364352), the most successful of these systems relied on the formation of alkali metal-ether complexes in order to provide a positive charge to the SDA.
In accordance with the present invention, there is provided a process for preparing a zeolite having the MEL crystal structure which comprises:
(a) preparing an aqueous solution comprising a source of an oxide selected from the group consisting of oxides of silicon, germanium or mixtures thereof; and a 2,2-diethoxyethyltrimethylammonium cation;
(b) maintaining the aqueous solution under conditions sufficient to form crystals of a zeolite having the MEL crystal structure; and
(c) recovering the zeolite crystals.
The present invention also provides the above process wherein the aqueous solution further comprises a source of boron oxide.
Also provided is the above process wherein the aqueous solution further comprises a source of alkali metal oxide.
The present invention also provides a process wherein the aqueous solution further comprises a source of boron oxide and a source of alkali metal oxide.
Further provided by the present invention is a crystalline material composition, as-synthesized and in the anhydrous state, having the MEL crystal structure and whose general formula, in terms of oxide mole ratios is:
Q/YO20.03-0.05
where Y is silicon, germanium or mixtures thereof and Q is 2,2-diethoxyethyltrimethylammonium cation.
Further provided by the present invention is a crystalline material composition, as-synthesized and in the anhydrous state, having the MEL crystal structure and whose general formula, in terms of oxide mole ratios is:
Q/YO20.03-0.05
M+/YO20.0005-0.25
where Y is silicon, germanium or mixtures thereof; Q is 2,2-diethoxyethyltrimethylammonium cation and M is an alkali metal cation.
The present invention also provides a crystalline material composition, as-synthesized and in the anhydrous state, having the MEL crystal structure and whose general formula, in terms of oxide mole ratios is:
YO2/B2O350 or higher
Q/YO20.03-0.05
where Y is silicon, germanium or mixtures thereof and Q is 2,2-diethoxyethyltrimethylammonium cation.
The present invention also provides a crystalline material composition, as-synthesized and in the anhydrous state, having the MEL crystal structure and whose general formula, in terms of oxide mole ratios is:
YO2/B2O350 or higher
Q/YO20.03-0.05
M+/YO20.0005-0.25
where Y is silicon, germanium or mixtures thereof; Q is 2,2-diethoxyethyltrimethylammonium cation and M is an alkali metal cation.
In one embodiment the present invention comprises:
(a) preparing an aqueous solution comprising sources of oxides capable of forming zeolites containing the MEL crystal structure and an organic SDA comprising a 2,2-diethoxyethyltrimethylammonium cation;
(b) maintaining the aqueous solution under conditions sufficient to form crystals of said zeolite; and
(c) recovering the crystals of said titanium-containing zeolite.
The SDA useful in the present process is a 2,2-diethoxyethyltrimethylammonium cation (DEOTA) that is capable of acting as a SDA to form zeolites having the MEL crystal structure. The SDA has a molecular structure of the general formula: 
Xxe2x88x92 is an anion which is not detrimental to the formation of the titanium-containing zeolite, such as those described below.
The anion for the salt may be essentially any anion such as halide or hydroxide which is not detrimental to the formation of the zeolite. As used herein, xe2x80x9chalidexe2x80x9d refers to the halogen anions, particularly fluorine, chlorine, bromine, iodine, and combinations thereof. Thus, representative anions include hydroxide, acetate, sulfate, carboxylate, tetrafluoroborate, and halides such as fluoride, chloride, bromide, and iodide. Hydroxide and iodide are particularly preferred as anions.
The process of the present invention comprises forming a reaction mixture containing sources of an oxide of silicon, germanium or mixtures thereof (Y); an organic SDA comprising a DEOTA cation (Q); water; and, optionally, a source of boron oxide and/or a source of alkali metal oxide (M), said reaction mixture having a composition in terms of mole ratios within the following ranges:
The reaction mixture may be prepared using standard zeolite preparation techniques. Typical sources of silicon oxide include silica hydrogel, tetraalkyl orthosilicates, and fumed silica. A typical source of boron oxide is boric acid. The preferred alkali metals are sodium and potassium.
In preparing the MEL zeolites according to the present invention, the reactants and the templating agent are dissolved in water and the resulting reaction mixture is maintained at an elevated temperature until crystals are formed. The temperatures during the hydrothermal crystallization step are typically maintained at about 135xc2x0 C. It has been found that higher temperatures (e.g., 150xc2x0 C.) may cause decomposition of the DEOTA, and that lower temperatures (e.g., 115xc2x0 C.) can result in the formation of an amorphous product and/or layered products. The crystallization period is typically greater than about 20 days.
The hydrothermal crystallization is usually conducted under pressure and usually in an autoclave so that the reaction mixture is subject to autogenous pressure. The reaction mixture can be stirred during crystallization.
Once the crystals have formed, the solid product is separated from the reaction mixture by standard mechanical separation techniques, such as filtration. The crystals are water-washed and then washed with acetone and dried, e.g., at 90xc2x0 C. to 150xc2x0 C. for from 8 to 24 hours, to obtain the as-synthesized zeolite crystals. The drying step can be performed at atmospheric or subatmospheric pressures.
During the hydrothermal crystallization step, the crystals can be allowed to nucleate spontaneously from the reaction mixture. The reaction mixture can also be seeded with crystals of zeolites containing the MEL crystal structure both to direct and accelerate the crystallization, as well as to minimize the formation of any undesired crystalline phases. When seed crystals are used, typically 0.1% to about 10% of the weight of silica used in the reaction mixture are added.
Due to the unpredictability o f the factors which control nucleation and crystallization in the art of crystalline oxide synthesis, not every combination of reagents, reactant ratios, and reaction conditions will result in crystalline products. Selecting crystallization conditions which are effective for producing crystals may require routine modifications to the reaction mixture or to the reaction conditions, such as temperature, and/or crystallization time. Making these modifications are well within the capabilities of one skilled in the art.
The MEL zeolite products made by the process of this invention have an as-synthesized composition comprising, in terms of mole ratios in the anhydrous state, the following:
Q/YO20.03-0.05
where Y is silicon, germanium or mixtures thereof and Q is 2,2-diethoxyethyltrimethylammonium cation
When alkali metal is present, the MEL zeolite products made by the process of this invention have an as-synthesized composition comprising, in terms of oxide mole ratios, the following:
where Y is silicon, germanium or mixtures thereof; Q is 2,2-diethoxyethyltrimethylammonium cation and M is an alkali metal cation.
When boron oxide is present, the MEL zeolite products made by the process of this invention have an as-synthesized composition comprising, in terms of mole ratios in the anhydrous state, the following:
where Y is silicon, germanium or mixtures thereof and Q is 2,2-diethoxyethyltrimethylammonium cation.
When both alkali metal and boron oxide are present, the MEL zeolite products made by the process of this invention have an as-synthesized composition comprising, in terms of mole ratios in the anhydrous state, the following:
where Y is silicon, germanium or mixtures thereof; Q is 2,2-diethoxyethyltrimethylammonium cation and M is an alkali metal cation.